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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

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406
ALASKAN GLACIER STUDIES
During the summer of 1910 part of this margin was accurately mapped eight times, seven of the surveys being made by the railway engineers, and one by our topographer. The changes are shown in the accompanying table and in PI. CLIV.
RATES OP ADVANCE OF NORTHERN MARGIN OF CHILDB GLACIER IN 1910 AND 1911
Dates	Days	Advance, in Feet		Rate per Day, in Feet	
		Greatest	Average	Fastest	Average
June   3 to July  29, 1910	56	124	116	2.2	2.07
July  29 to Aug.    6	8	26	23	3.25	2.87
Aug.    6 to Aug. 11	5	41	8	8.2	1.60
Aug.  11 to Aug. 17	6	27	4	4.5	.66
Aug. 17 to Aug. 29	12	42	19	3.5	1.58
Aug. 29 to Sept. 19	21	37	27	1.76	1.28
Sept. 19 to Oct.    5, 1910	17	IS	7*	.7	.44
Oct.     5 to June 16, 1911	254		97	-----	.38
This striking rate of advance, averaging two to eight feet a day, shows variations in velocity with two maxima in the average movements. The fastest movements show a very rapid increase in rate of advance to the maximum August 11, and an unusually rapid decrease again.
It was a rare opportunity to witness the visible forward movement of this margin of Childs Glacier into the forest, as we did during daily visits about the middle of August. A series of lobes developed, though some of them were not persistent (PL CLIV), and at the ends of these lobes the day-to-day changes were most pronounced. Enormous ice blocks rolled down the frontal slope, some of them sliding many feet into the forest. Trees were overturned. Turf and grass were ploughed up and carried on the glacier (PI. CXLV, B), whose ice blocks acted as ploughshares, which ripped up turf and shrubs and carried them along ten or fifteen feet higher than the level at which they grew on the plain. Gravel and turf were heaped up in a discontinuous terminal moraine. Yet one saw and heard little of a spectacular nature while traversing the ice front. It was an irresistible, steady movement; but slow, as the mills of the gods. As impressive as anything was to find tons of ice resting where one stood to take a photograph the day before, or to find some great tree 100 years old prone on the ground with the butt beneath the glacier, where the day before the tree was upright, with the ice just touching it, or with a space of from one to three feet between the glacier and the tree. In places ice blocks, after sliding forward, lay tilted at all conceivable angles on top of the push moraine; but this seemed to be clearly overriding, rather than sub-marginal accumulation.
Near the river, the ice was thickly covered with ablation moraine and had relatively few crevasses in August, 1909; but photographs taken during the winter of 1909-1910 and early in the spring of 1910, show it severely riven by cracks. In August, 1910, practically all the moraine had slid into the crevasses, though a few small stones still